If you're working on .conf files , make sure that you're editing the partition where the Routing Domain is configured.
load sys config command parses all text configuration files in all partitions, so my main suspect will be that there might be an object configured in the wrong partition referencing a Routing Domain ID that it cannot see.
If the RD is in a partition, remember that objects in /Common/ can't see it, so if you edited /Common/ partition bigip_base.conf file to insert a self-ip referencing RD 66 it will show a similar error.
Check it like this:
grep -i %66 /config/bigip*.conf
Also, remember that Partition specific configuration files are in /config/partitions/<name>/bigip*.conf , again I would check those too.
If you need to quickly un-do .conf file edits, you can just run a "tmsh save sys config" that will overwrite .conf files.
Also, when wirking directly on .conf files, I'd recommend creating a .bkp copy before making any edit
Hope this helps
CA